In plants for the manufacture of wet-process phosphoric acid ground phosphate rock is reacted with sulfuric acid. The reactor slurries contain ground rock and typically, about 33% phosphorous pentoxide (45% phosphoric acid), 2-5% sulfuric acid, 1-3% fluorosilic acid, fluorosilicates and small amounts of chlorides and hydrofluoric acid. The operating temperature is typically about 80.degree. C. As a result of these conditions of operation the reactor slurries are quite abrasive as well as exceedingly corrosive to metals.
In wet-acid plants castings need to be resistant to the corrosiveness and abrasiveness of the reactor slurries. Typically, alloys for such service would be nickel-chromium-iron alloys containing significant amounts of carbon. However, because castings for pumps and other equipment are often of several tons weight and several inches section thickness, various difficulties are encountered in the foundry when attempting to remove the gates and risers from such large and heavy section castings. One of these difficulties is due to the fact that high chromium alloys cannot be cut with oxyacetylene cutting torches. Another problem is due to the fact that very large risers are required to feed heavy, massive castings during the freezing stage but, at present, sufficiently large and maneuverable cutting wheels to remove such large risers are unavailable. Also, removing very large risers from any brittle alloys by cutting with an arc air torch tends to cause cracking in the castings starting at the riser neck unless the cutting is done very slowly. Another method of accomplishing riser removal has been to design the riser neck to have a notch or indentation ring of reduced diameter and to break the riser off with a blow from a sledge hammer, but this technique does not work for every casting design. Alternatively, the riser neck is notched by a fairly shallow ring cut with an arc air torch followed by breaking off the riser with a sledge hammer blow. However, either of these latter techniques may fail either due to cracking the casting or to a "break-in" at the casting area adjacent to the riser, that is, some portion of the casting volume itself adheres to the riser and thus leaves a void in the casting proper when the riser is removed. Therefore, for very large castings or for castings whose designs tend to cause "break-ins" some sacrifice of casting abrasion resistance, and therefore, reduced brittleness, is warranted in exchange for the ability to produce such castings in the foundry.
Furthermore, since silicon will react with fluorine most phosphoric acid wet-process plants attempt to minimize the highly corrosive free fluoride ion concentration by keeping a high silica to fluoride ion ratio in the reactor, usually by adding more silica sand. However, in some plant operations and with some phosphate rock deposits this is not always accomplished, in which case free fluoride ion, especially if above a concentration of about 2000 ppm, readily attacks high silicon metallic alloys which, as noted above, are often employed because of their abrasion resistance in certain other applications.
Because of these various problems there remains a need for alloys which combine sufficient toughness for good founding properties and which have good abrasion and corrosion resistance to high fluoride ion-containing wet-process phosphoric acid reactor slurries.